BCS: 50 Years

Superconductivity routinely vexed the most accomplished theoretical physicists for almost half a century after H. Kamerlingh Onnes first documented the phenomenon in his Leiden laboratory. The Bardeen-Cooper-Schrieffer theory of superconductivity, which appeared in 1957, surmounted decades of frustration and garnered immediate acclaim. Its larger legacy would emerge gradually as expansions and applications of the theory cemented its relevance for a broad range of physical phenomena. BCS: 50 Years, edited by Leon Cooper and Dmitri Feldman, unites 23 contributions from a range of physicists, many of whom contributed to the early development and application of BCS theory. Taken together, these essays represent a valuable first step towards understanding the manifold aspects of the BCS legacy.

The volume's first section establishes the historical background of BCS with contributions from each of its architects. Leon Cooper, J. Robert Schrieffer, and John Bardeen recall their personal intellectual journeys leading to the theory's formulation, each pointing to a different critical factor. Cooper observes that isolating one qualitative feature of the superconducting state—the distinctive energy gap below Tc—led to the discovery of a new ground state for interacting electrons. Schrieffer's account opens a window into the geography of collaboration at the University of Illinois: Cooper shared an office with Bardeen, but also spent much of his time at the “Institute for Retarded Studies,” the lively graduate student offices where Schrieffer had his desk. Bardeen, characteristically sharing recognition for his accomplishments, credits BCS to the robust interplay between theory and experiment that guided their work.

These narratives are not contrasting so much as complementary. Each isolates an element of the mid-1950s intellectual context in which BCS emerged that illuminates the story in a slightly different way. The volume's other historical recollections further detail to that picture. Slichter, reinforcing Bardeen's claim that understanding superconductivity required rich theory-experiment dialogue, discusses how the experimental underpinning of BCS intertwined with contemporaneous developments in NMR. Pines recalls that the political climate during the development of BCS was just as frenzied as the intellectual climate. Through a careful reconstruction of several unsuccessful theories of superconductivity, Schmalian develops a compelling account of how “failed” efforts in science build the scaffolding on which future successes rest. Gor'kov and Anderson show how BCS immediately generated further advances: Gor'kov recounts the rapidity with which the theory took hold in the Soviet Union, leading to its reinterpretation with QFT methods, and Anderson describes how BCS paved the way for the appreciation of electron-electron pairing mechanisms, which undergirds current understanding of superconductivity as a single phenomenon that comes in two manifestly different flavors.

With the historical background in place, the rest of the volume tours the full and colorful variety of ways BCS motivated subsequent theoretical and experimental work. Clarke's description of experimental developments and applications of SQUIDs is particularly engaging. Adjacent articles from Chu and Abrahams describe the evolution of high-Tc superconductivity from the experimental and theoretical angles, respectively. It is a testament to the editors' care that they introduce these two pieces with Cohen's thorough overview, including historical background, of BCS's utility for understanding critical temperatures. These articles come as part of a sequence outlining the state-of-the-art in superconductivity research, a category in which the contributions of Fulde, Goldman, and Le Doussal also belong. Linking these works with insights from the book's historical overview imparts a distinct sense of the theory's enduring impact.

The scale of BCS's import for twentieth-century physics comes into sharpest focus through the papers discussing its broader applications. Leggett reviews how BCS and the phenomenon of Cooper pairing proved useful, often in unexpected ways, for superfluidity research—a sentiment echoed in Ketterle, et al., Halperin, et al., and Zwicknagl and Wosnitza. The full breadth of BCS's influence is most evident in the form of contributions to this volume from high energy, nuclear, and astrophysicists. Baym describes the role of BCS in aiding understanding of neutron stars and high-density quark matter and Nambu, Weinberg, and Wilczek each acknowledge the high energy community's debt to BCS for highlighting the importance of spontaneous symmetry breaking.

Readers of BCS: 50 Years should be aware that many contributions are highly technical. Even so, the contents are rich enough that historians and physicists of all stripes will find much of value in its pages. Aside from the omission of the original BCS paper, the inclusion of which would have added a useful reference point, the primary shortcoming of this volume is that much of its content is available elsewhere. Mitigating this criticism is the fact that the juxtaposition of these chapters, not their contents alone, makes the volume well worth the price. Like superconductors themselves, the book gains its most striking and useful properties from the interaction between its component parts. Superconductivity—and, indeed, condensed matter physics as a whole—is ripe for concentrated historical attention. Just as BCS acted as a powerful motivator for further physical research, the descriptions of the theory and its influence contained within BCS: 50 Years furnish a valuable foundation on which historians may build.

Joseph D. Martin is a Ph.D. Candidate in the University of Minnesota's Program in the History of Science, Technology, and Medicine and a Dissertation Writing Fellow at the Philadelphia Area Center for History of Science.

The articles in this issue represent the views of their authors and are not necessarily those of the Forum or APS.